The synthesis of higher order silanes, SinH2n+2 (where n≧3), has been accomplished through many synthetic approaches. These include: (1) treatment of metal silicides that contain ionic silicon-metal bonds with protonic reagents such as aqueous acids; (2) reaction of protonic reagents such as NH4Br in liquid ammonia; (3) reduction of chlorosilanes, SinCl2n+2 (n=2-3), with hydridic reagents such as LiH, NaH, NaBH4, LiAlH4, etc . . . ; (4) reduction of silicon using hydrogen sulfide or a metal sulfide catalyst; (5) reaction between a silicon-oxygen compound containing Si—H or Si—Si bonds and a hydride, an alkoxide or an amalgam of an alkali metal; (6) reaction of higher order partially halogenated silanes and alkalai metal silyl salts; (7) electric discharge in monosilane gas mixtures′ (8) dehydrogenation condensation of hydrosilanes using a platinum-group metal complex or Lanthanide-group metal compounds as a catalyst; and (9) pyrolysis of monosilane and higher order silanes with and without catalysts.
The majority of these synthetic methods suffer from numerous problems that will have to be solved before cost-effective, safe commercial synthesis of these valuable compounds can be achieved.
The use of aqueous acid solutions results in limited yields due to hydrolysis of the products as they are formed. This results in the production of large quantities of monosilane and low yields of higher order silanes. Liquid ammonia syntheses of this nature are more effective due to elimination of these hydrolysis reactions, but commercial-scale synthetic reactions are difficult to implement. See N. N. Greenwood and E. A. Earnshaw, Chemistry of the Elements (199*).
The use of hydrogen sulfide or a metal sulfide as a catalyst is problematic from safety and final product purity considerations. That is, hydrogen sulfide is a highly toxic, high-pressure gas and it is difficult to separate it from higher order silane compounds.
Reduction reactions of higher order chlorosilanes, SinCl2n+2, are highly effective on a laboratory scale, but the starting chlorine compounds are difficult to synthesize and expensive. These reactions typically involve the combination of highly flammable ether solvents and flammable hydridic reducing agents and can be difficult to control when synthesizing on larger scales. Furthermore, the reactions can be very dangerous from the perspective of fire hazards and the separation of the desired silane compound(s) from the solvent and byproducts can be difficult.
Reactions involving silicon-oxygen compounds that contain Si—H or Si—Si bonds and a hydride, an alkoxide or an amalgam of an alkali metal, an alkali metal or alkali metal hydride require the handling of large amounts of hazardous chemicals. The combination of chemicals required presents a severe fire hazard during synthesis, particularly on a commercial scale.
Reaction of partially halogenated higher order silanes and alkali metal silyl salts requires costly synthetic intermediates and involves the use of highly reactive chemicals in ether solvents. As a result, this method presents serious fire hazard issues.
Prior art electric discharge methods that rely upon the use of monosilane and reduced pressure processes are expensive and typically result in low yields of higher order silanes, SinH2n+2 (n>2), with a large percentage of the starting materials being converted to amorphous hydrogenated silicon particles. More recently, an effective method for converting monosilane into disilane using silent electric discharge has been demonstrated, but trisilane was the only observed higher order silane and only in very low yield (around 1%). See U.S. Pat. No. 5,505,913.
U.S. Pat. No. 4,792,460 discloses the production of polysilanes and polygermanes, including disilane and digermane, using electric discharge methods for use in in-situ deposition reactions for amorphous hydrogenated materials. This prior art reference discloses the use of atmospheric, and higher, pressure discharge reactions to generate mixtures of polysilanes and polygermanes from monosilane and monogermane expressly for the purpose of using them in in-situ deposition reactions that do not require isolation of the chemical species. This method precludes use of pure polysilanes or polygermanes, particularly liquids (i.e., SinH2n+2 where n≧3 and GenH2n+2 where n≧2), in deposition reactions and further limits the application of the mixtures obtained to atmospheric pressure. The apparatus employed does not allow isolation, purification and collection of the higher order polysilanes and polygermanes.
In the catalytic methods of the prior art, the catalytic activity of the platinum-group and Lanthanide compounds is typically low, the reactions require extended periods of time and the yield of the desired higher order silanes is typically low. Furthermore, the cost of the starting materials of these reactions is typically high and contamination of the final products with unwanted metal impurities can be a serious issue.
Methods of producing higher order silanes from the pyrolysis of monosilane (see U.S. Pat. No. 6,027,705) are effective in producing a broad range of higher order silanes. These are formed as a liquid mixture that must be distilled to isolate the desired product(s) from one another. Traditional distillation techniques require heating of the mixture, resulting in the thermal decomposition of higher order silanes during the distillation. Furthermore, the method requires multiple, lengthy reaction steps, is not overly efficient and requires a complex apparatus. As a result, the commercial maturity of this technique is not sufficient to provide large quantities of pure, higher order silanes.
The lack of an efficient, safe and cost effective method for synthesizing, isolating and purifying higher order polysilanes and polygermanes, with a high degree of purity and on an industrial scale, hinders their use in semiconductor processes. As a result, there remains a need for such a process.